A non-invasive method for pathological anatomy (e.g., tumor, legion, vascular malformation, nerve disorder, etc.) treatment is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a pathological anatomy site from multiple angles, with the patient positioned so the pathological anatomy is at the center of rotation (isocenter) of the beam. As the angle of the radiation source is changed, every beam passes through the pathological anatomy site, but passes through a different area of healthy tissue on its way to the pathological anatomy. As a result, the cumulative radiation dose at the pathological anatomy is high and the average radiation dose to healthy tissue is low. The term radiotherapy refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centi-Gray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted by the magnitude of the radiation.
Traditionally, medical imaging was used to represent two-dimensional views of the human anatomy. Modern anatomical imaging modalities such as computed tomography (CT) are able to provide an accurate three-dimensional model of a volume of interest (e.g., skull or pathological anatomy bearing portion of the body) generated from a collection of CT slices and, thereby, the volume requiring treatment can be visualized in three dimensions. More particularly, in CT scanning numerous x-ray beams are passed through a volume of interest in a body structure at different angles. Then, sensors measure the amount of radiation absorbed by different tissues. As a patient lies on a couch, an imaging system records x-ray beams from multiple points. A computer program is used to measure the differences in x-ray absorption to form cross-sectional images, or “slices” of the head and brain. These slices are called tomograms, hence the name “computed tomography.”
During treatment planning, a volume of interest (VOI) from anatomical (e.g., CT) and/or functional imaging is used to delineate structures to be targeted or avoided with respect to the administered radiation dose. A volume of interest (VOI) may be defined as a set of planar, closed polygons, as illustrated in FIG. 1A. The coordinates of the polygon vertices are defined as the x/y/z offsets in a given unit from the image origin. Once a VOI has been defined, it may be represented as a bit wise mask overlaid on the functional and/or anatomical image (so that each bit is zero or one according to whether the corresponding image volume pixel (voxel) is contained within the VOI represented by that bit), or a set of contours defining the boundary of the VOI in each image slice. Conventional VOI imaging architectures may utilize a three-tier representation structure: VOI-contourslice-contour. FIG. 1B illustrates the three-tier VOI structure in a Unified Modeling Language (UML) graph with a sample VOI.
One problem encountered in external beam radiation treatment is that pathological anatomies (e.g., a tumor) may move during treatment, which decreases accurate target localization (i.e., accurate tracking of the position of the target). Most notably, soft tissue targets tend to move with patient breathing during radiation treatment delivery sessions. Respiratory motion can move a pathological anatomy in the chest or abdomen, for example, by more than 3 centimeters (cm). In the presence of such respiratory motion, for example, it is difficult to achieve the goal of precisely and accurately delivering the radiation dose to the target, while avoiding surrounding healthy tissue. In external beam radiation treatment, accurate delivery of the radiation beams to the pathological anatomy being treated can be critical, in order to achieve the radiation dose distribution that was computed during the treatment planning stage.
Conventional methods for tracking anatomical motion utilize external markers and/or internal fiducial markers. Such conventional methods do not enable modeling of the anatomical change due to the respiratory cycle using conventional VOI imaging architectures. Moreover, such conventional methods do not take into account non-rigid motions and deformations of surrounding anatomy, as a function of motion cycle.